finding things when walking

make leaf art on the ground
make stone stacks (cairns)
in the fall, organize leaves by color, making gradients, rainbows, and color wheels (as best you can)

collect river rocks
catch small fish, insects and butterflies; study and release them
catch tadpoles; take them home to raise to polliwogs and frogs
make leaf rubbings
leave sugar grains or bread crumbs near an anthill and watch the ants take the food into their nests

look at birds, squirrels or other far away animals through binoculars
identify bird calls
use a scavenger hunt list and make a collection of items to take home
use your shadows to enact silly plays; use what you find in nature as props
make fairy homes